Receptor tyrosine kinases (RTKs) mediate cell growth and differentiation in many animal tissues. Twenty different classes of RTKs are found in humans and include receptors for insulin, epidermal growth factor (EGF), and vascular endothelial growth factor (VEGF). RTKs consist of an extracellular ligand-binding region, a single membrane-spanning region, and a cytoplasmic tyrosine kinase. Ligand binding to the extracellular region triggers activation of the kinase activity, which leads to phosphorylation of downstream targets and initiation of a signaling cascade. Inappropriate activation of RTKs can lead to uncontrolled cell growth, and alterations in 28 of the 58 human RTKs have been associated with some form of cancer. For example, overexpression of the EGF receptor homologue ErbB2/HER2 is found in 20-25% of human breast cancers and is associated with more aggressive tumors and a poorer prognosis. Inhibiting RTKs has proven an effective anticancer strategy, and monoclonal antibodies against the EGF receptor (Erbitux) and ErbB2/HER2 (Herceptin) have been approved for the treatment of colorectal and breast cancer, respectively. My laboratory has recently developed methods that simplify expression of cysteine-rich glycoproteins, which includes most RTK extracellular regions, for X-ray structural studies. These methods have enabled us to determine crystal structures of the extracellular regions of all four members of the human EGF receptor (EGFR) family as well as ErbB2/HER2 complexed with the Fabs of Herceptin and another therapeutic antibody, Omnitarg. Combined with results from other labs, these structures have led to new models of signaling for the EGFR family and provided insight into the mechanism of anticancer agents targeting this family. Our first aim is to extend these studies and determine crystal structures of the extracellular regions of specific pairs of EGFR family members with bound ligand to better understand the nature of signaling and specificity within this family. Our second aim is to perform biochemical and structural studies of two downstream effectors of signaling by EGFR family members to better understand how EGFR generated signals are transmitted and regulated within the cell. Our third aim is to initiate structural studies of two related classes of RTKs, which include the receptors for macrophage colony-stimulating factor (MCSF), stem cell factor (SCF), Flt3 Ligand, and VEGF, to better understand their function in normal and disease states.